A stacked-sheet prefeeder normally comprises a sheet guiding device for receiving therein a comparatively large stack of sheets, and a device for producing a scaled stream, which removes from the lower surface of the stack of sheets held in the sheet guiding device sheets in the form of a stack or in a scalelike mode of arrangement and which supplies the resultant scaled stream to an intermediate tray from which individual sheets are removed by a separation device so that they can be processed subsequently. It follows that such a scaled-stream production device is normally a component of a two-step separation principle in the case of which a scaled stream is first formed from a large stack of sheets, the scaled stream being then, finally, converted into a stream of paper consisting of separate sheets. Such a scaled-stream production device comprises, typically, a feed roll engaging a stack of sheets on the lower surface of the entire stack, which is held by the sheet guiding device, and transporting the stack of sheets in the direction of the intermediate tray. For limiting the height of the stack, a stack height limitation device is typically arranged in the area of the lower edge of the entire stack; the typical components of the stack height limitation device are a rotating roll and a stationary roll, which is arranged at a fixed distance from the rotating roll and the free distance of which delimits the height of the substacks removed from the lower surface of the entire stack. The scaled stream produced by this type of device is often irregular with regard to the scale distances and the scaled-stream thickness.
DE-A 29 30 270 discloses a means for detecting irregularities in the sheet supply to a sheet-processing machine, which is provided with a measuring device for detecting a size that increases as the number of superimposed sheets increases and an evaluation means with the aid of which an electric signal can be supplied if irregularities should occur. FIG. 1 shows a feeder with a stack to be fed from which the sheets are removed by means of a suction device and are fed to the printing machine via a feed table and alignment devices. Prior to the feed table, a feed roll is provided, which can be lifted by means of a suitable control so that a respective new sheet can be introduced. This feed roll is spring urged towards a lower roll. The distance between these rolls corresponds to the thickness of the sheets disposed between the rolls, the distance being measured by a distance sensor. Suitably weighted or averaged output signals of the distance sensor are compared with a stored reference value so that a double sheet or an imperfect sheet can be identified.